Currently, a plurality of sensors already exist which acquire information in and around the vehicle and make this information available for further processing. These include, among others, sensors for the acquisition of dynamic objects in the vehicle environment. As a rule, the information obtained in this manner contains a vehicle-specific timestamp. A timestamp is required in order to mutually merge the results of the different sensors. Assistance functions also require a uniform time base in all components for the purpose of homogenizing and, as a whole, utilizing the information of several separate system components (different vehicles, infrastructure devices, etc.)
Only in this manner can a temporal relationship be established between data of different system components. One corresponding example is the perception relating to all vehicles by which environment models of vehicles and infrastructure units are communicated to other traffic participants and are associated and merged there with the results of the local environment acquisition. When information from a vehicle or from infrastructure units is transmitted to other communication partners (Car2X), existing methods, as a rule, use NTP-based time bases (such as NTP for the synchronization between server clusters) or the global timestamp of a GPS receiver. A GPS timestamp, as provided by a GPS receiver, under optimal conditions, will indicate the measuring point in time of the satellite signals with a precision of a few nanoseconds. However, because of the receiver-internal processing time, similarly to the above-mentioned sensors, this timestamp will only be available after an, as a rule, fluctuating processing time. This processing time may range from up to 100 ms to more than 100 ms. When this timestamp is used for synchronizing other units, this processing time will result in an equally high uncertainty. For example, at a vehicle speed of 30 m/s, this results in an error of more than 3 m. Furthermore, the GPS timestamp is only available with an updating rate of 1 to 20 Hz.
The performance of a time synchronization by means of an NTP server is, among other things, coupled to the efficiency of the basic operating system of the client to be synchronized (for example, Windows) and to the utilization of the server network. As a function of the time behavior of these two components, different uncertainties and time delays may arise. In order to synchronize two computers (NTP server and NTP client), which are mutually connected by a network (for example, the Internet), the NTP will measure the roundtrip delay of packets between the NTP server and the client. The NTP is based on the assumption that the transmission latency between the server and the client has the same length and is therefore in each case half the roundtrip delay.
However, in connection with the above-mentioned solutions, it is problematic that the availability for the time synchronization is not sufficient for security applications. Thus, for example, a GPS-based time base cannot be synchronized in the case of a system start in a blocked area (for example, in an underground garage or a street canyon between tall buildings). On the other hand, NTP servers, which synchronize themselves, for example, by way of the power supply system, are susceptible to power outages.
It is therefore an object of the present invention to eliminate the above-mentioned problems at the signal exchange between two mobile units. For this purpose, a method is provided for time-stamping a first message of a first mobile unit to a second mobile unit. In addition, a corresponding mobile unit is provided. According to the method of the invention, a roundtrip time is determined between the first mobile unit and a base station. The roundtrip time relates to that communication path that is provided for the first message. The determination of the roundtrip time can take place in the base station, which base station is further developed, for example, as part of a terrestrial mobile communication system. Such base stations are, for example, called “NodeB” or “eNodeB”. After the roundtrip time has been determined, the first message sent by the first mobile unit will be received in the base station. This message may, for example, contain information that is based on sensor signals of the first mobile unit. Such sensor signals may, for example, represent collision-relevant information which is forwarded to additional mobile units by way of the base station. Since, as initially mentioned, a common time base of the mobile units is required or information for the integration of the content of the message in a common time context, according to the invention, while taking into account the roundtrip time, a timestamp is added to the first message. The timestamp may be added by using a time base determined within the base station.
In particular, the taking into account of the roundtrip time takes place such that the timestamp permits a conclusion as to the point in time at which the first message or the information contained therein had been created. When the time-stamped first message is subsequently sent to the second mobile unit, the second mobile unit can check and correspondingly interpret the contained data with respect to their age by using an own time base. The second mobile unit can, for example, extrapolate sensor data as well as a vehicle speed, which are contained within the first message, in order to estimate a change of the data while taking into account the travel time of the first message. For example, collision-relevant traffic situations can be interpreted by way of speed and range information in the first message by means of the timestamp and can therefore be processed for creating a more precise image of the vehicle environment.
The present invention therefore has the advantage, among others, that information exchanged between all involved vehicles concerning environment information and traffic situations has a higher added value because it permits credible conclusions for the recipient.
Preferably, the roundtrip time can already be determined in the base station, before the receiving of the message sent by the first mobile unit, by way of a second message sent from the base station and/or from the first mobile unit. This can, for example, take place cyclically in order to always have a suitable roundtrip time available between a base station and a respective mobile unit for a possibly required time-stamping. The second message for determining the roundtrip time may, for example, be a simple ping, as used between the NTP server and the NTP client in computer networks for determining a transmission latency. It is, for example, also contemplated that a respective mobile unit cyclically determines a roundtrip time and adds the roundtrip time when sending the first message to the base station. In this manner, by means of its own time base in connection with the roundtrip time, the base station can add a suitable timestamp, in which case, in the base station, particularly the information for the roundtrip time can be exchanged against the timestamp, whereby transmission bandwidth is saved. When the roundtrip time is determined only in an event-based manner, thus only just before a sensor signal is to be sent as a first message, communication expenditures by cyclical roundtrip determination can additionally be avoided.
The first mobile unit and the second mobile unit may be developed as passenger cars, as unmanned transport units, as watercraft or aircraft. In particular, vehicles authorized to be driven on roads (irrespective of their drive concept) are addressed, because the unpredictably changeable conditions with respect to the communication infrastructure available to them make a time-stamping by a base station seem advantageous without these limitations. Even when a vehicle as the mobile unit is shielded in an underground garage from satellite-based locating signals and therefore has no access to a global time base, the sending of a sensor signal on the basis of the present invention can provide sensor signals, for example, by way of terrestrial mobile communication infrastructures, which can be analyzed by additional mobile units.
Information, which is sent by the first mobile unit in the form of the first message to the base station or to a second mobile unit, may be determined, for example, by use of ultrasound, lidar, radar or laser sensors. In addition or as an alternative, the first message may also contain speed or location information of the first mobile unit which, after a delayed reception by a second mobile unit, can be appropriately interpreted on the basis of the timestamp.
In order to carry out the time-stamping within the base station in a particularly reliable and exact manner, the base station may be equipped for receiving time signals concerning different communication paths from the same or from different time sources. For example, a first signal can be determined by a receiver of a satellite-based locating system. This receiver may have a higher-value design than receivers used in mass-produced articles (such as vehicles, navigation systems, smartphones, etc.). In this manner, a highly accurate interpretation of the time base of one or more satellites becomes possible.
For consolidating the time signal, a second time base is received, for example, by way of a terrestrial mobile communication system (GSM, EDGE, UMTS, LTE LTE-A, etc.) or by way of a local access point (such as an access point of a WiFi network). Theoretically, other globally available systems with a known time behavior may also be used. Naturally, signals received from additional transmitters or sources can also be taken into account for creating the time base. On the basis of the available time base signals, a time base is determined for the base station, and the first message is provided with a timestamp by using the time base. For determining the time base from several signals, for example, a maximal-value function can be applied to time signals received within a predefined time period. When, for example, within an interval of 10 ms, the signal of two different sources is received, during the subsequent creation of the time stamp, that time signal can be used that has the higher amount. The present time signal having the highest amount is probably subjected to the lowest transmission latency, so that it represents the most reliable starting point for providing the time base. In this manner, the respective time base having the lowest tolerance can be established particularly in the case of fluctuating transmission conditions of different possible time signals. Also the time stamp and the information contained in the first message therefore have the highest possible quality.
According to a second aspect of the present invention, a mobile unit is provided which is further developed particularly as a road vehicle (for example, a passenger car). The mobile unit comprises a sensor which is implemented, for example, as an environment sensor. Concerning the operating principles of the environment sensor system, reference is made to the examples described in connection with first-mentioned aspect of the invention. In addition, the mobile unit comprises a send-receive (transceiver) unit which can, for example, exchange wireless signals with a base station and/or with other mobile units. According to the invention, the mobile unit is equipped for picking up a collision-relevant environment signal by use of sensors, which environment signal is potentially of interest also to a second mobile unit. Furthermore, by way of the send-receive unit, the mobile unit is able to carry out a roundtrip time determination while interacting with a base station and supporting this roundtrip time determination. The send-receive unit also has the purpose of sending the signal picked up by use of the sensor and possibly a determined roundtrip time by way of the base station to a second mobile unit. The characteristics, the combination of characteristics and the advantages resulting therefrom correspond to those indicated in connection with the first-mentioned aspect of the invention such that reference is made to the previous statements in order to avoid repetitions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.